1. Field of the Invention
The present invention relates to an optical reproducing device, and more particularly, to a wobble signal reproducing device for reproducing a wobble signal recorded on writable optical discs.
2. Description of the Related Art
Optical discs are classified into once-writable discs such as CD-R, DVD-R and the like, and re-writable discs such as CD-RW, DVD-RW, DVD-RAM, and the like, in addition to read-only discs such as CD-ROM and DVD-ROM. Unlike such existing read-only discs, as for the re-writable discs, a channel clock signal cannot be generated from a reproducing signal. Thus, the re-writable discs have grooves wobbled in a certain period having information, and a wobble signal is detected so that a channel clock signal necessary for writing data is generated.
Furthermore, the CD/DVD read/write modes are divided into the constant linear velocity (CLV) mode and the constant angular velocity (CAV) mode depending on a spindle motor control method, and, in general, the CLV mode is employed for a low speed operation, and the CAV mode is employed for a high speed operation. The CLV mode causes the spindle motor to spin fastest in the innermost of a disc and to spin slowest in the outermost of the disc, and has the same frequency characteristics in the innermost and outermost of the disc. However, the CAV mode spins the spindle motor at a constant speed regardless of which portion of the disc information is accessed from, and the innermost and outermost of the disc has a 2.4 times difference in frequency.
FIG. 1 is a block diagram for showing a conventional wobble signal reproducing device, and FIGS. 2A-2C explain a wobble signal reproducing process for the wobble signal reproducing device shown in FIG. 1.
As shown in FIG. 1, as for the light amount corresponding to the wobble signal measured in the portions A, B, C, and D on the disc, the adders 11 and 13 add the light amounts corresponding to the portions A and D and the light amounts corresponding to the portions B and C, and, as shown in FIG. 2A and FIG. 2B, the adders 11 and 13 output a (A+D) signal and a (B+C) signal, respectively, that have an RF signal in phase and a wobble signal in 180° phase difference. The (A+D) signal and the (B+C) signal are filtered through high-pass filters (HPFs) 21 and 23 each having a cut-off frequency sufficient to pass the wobble signal, so that a DC offset is removed.
Next, automatic gain controllers (AGC) 31 and 33 maintain the amplitude of the RF signal constant, so as to prevent the RF signal from being missed due to the wobble signal.
The subtractor 41 performs the subtraction between the gain-controlled (A+D) and (B+C) signals, and outputs a wobble signal as shown in FIG. 2C. The narrow-band pass filter (BPF) 45 filters the output wobble signal to detect a wobble signal having a high signal-to-noise (S/N) ratio, the automatic gain controller 47 amplifies its gain, and the comparator 49 quantizes the wobble signal, and then outputs a wobble clock signal.
The above existing wobble signal producing device has the band-pass filter 45 consisting of an active RC filter for a center frequency, and varies the center frequency of the band-pass filter 45 under controls of a system controller according to the variations of read/write speeds. The above control approach of the band-pass filter 45 has a disadvantage that the system controller controls the band-pass filter 45 every time the speed changes.
Furthermore, with signal reproducing speeds increasing, the above approach has a problem of deviating the pass band of the band-pass filter owing to an access limit of the system controller in case that the wobble signal is reproduced in the CAV mode. Thus, this causes the S/N ratio of the wobble signal to be reduced as well as the jitter in the wobble clock signal to be increased.